The role of miniature, lithographically produced devices in present day technology is well understood. Without integrated circuits--without semiconductor integrated circuits--the story would be quite different. Extension of the technology to incorporate other types of elements--in particular optical elements--is already having an impact.
Fabrication techniques which have brought us to our advanced state, by analogy to conventional photographic processes, depend upon actinic "resist" materials, which like photographic emulsions, yield patterned layers as defined by selective exposure to radiation. Resist chemistry has developed apace. Both positive and negative resists of required sensitivity to the many types of delineating radiation contemplated have emerged. Resists of necessary stability, sensitivity, wavelength selectivity, etc. to accommodate visible, infrared, ultraviolet, e-beam and ion beam radiation are available. Present ability to reliably produce high yield, longlasting circuitry, of critical dimension of a few micrometers and even to one or a fraction of a micrometer, is heavily dependent on the present advanced state of the resist and associated processing chemistry.
For the most part, pattern delineation of resist layers on devices undergoing fabrication has depended upon already-patterned masks, generally self-supporting masks, which pattern flood radiation. Beam writing has also played a role. While its chief significance has been in the fabrication of masks, for example by use of the Electron Beam Exposure System, it has also been used in direct fabrication of devices. Beam writing, avoids use of a mask and makes use of a focused beam, for example of accelerated electrons, to result in changed solubility of the resist to the developer. Another process, ion beam milling avoids the resist as well as the mask and is dependent upon direct physical removal of material through use of relatively high beam energy.
Other developments have brought about sophisticated advances concerning the nature of the device-functional material to be delineated. A variety of processes including Molecular Beam Epitaxy in its various forms, see "Molecular Beam Epitaxy", M. A. Herman, H. Sitter, Springer-Verlag, 1989 and Metal Organic Chemical Vapor Deposition, see J. P. Hirtz, M. Razeghi, M. Bonnet and J. P. Duchemin in "GaInAsP Alloy Semiconductors", ed. by T. P. Pearsall, J. Wiley and Sons, 1982, have emerged to yield extremely well-controlled layers having uniform thicknesses as small as a fraction of a wavelength of radiation within and beyond the visible spectrum. These and other sophisticated processes have been responsible for diminishing dimensions corresponding with line rules as small as fractions of a micrometer.
Lithographic procedures that have served well to this time are not optimal for new device generations. Dimensional control realized by substitution of shorter wavelength delineating radiation--deep ultraviolet or x-ray--is sometimes thwarted, at least in terms of lessened yield, by use of present-day lithographic processing. Problems include contamination as well as damage. A problem, growing in consequence, is crystallographic damage to material being patterned both in terms of such material itself and as propagated in subsequently overgrown material.
It is known that MBE processing is particularly susceptible to contamination resulting from resist residue, as well as residue resulting from resist processing. Chemical residue, necessarily removed before deposition of overlying MBE material, must be completely removed to realize the implicit advantages of the deposition process.
Other forms of contamination are of consequence. Efforts toward finer wavelength radiation--toward x-ray--are complicated by contamination, and sometimes the oxidation itself as resulting from exposure to air atmosphere. Most promising studies based on projection optics have been in the "soft" x-ray spectrum--e.g. 100 .ANG.-200 .ANG. wavelength--at which, dust particles cause a problem--usual carbon- or silicon-containing dust is opaque to x-ray in this wavelength range.
In short, contamination resulting from resist residue, previously of little consequence, as well as problems due to dust particles which may penetrate all but the most sophisticated filtering systems, point to a need for resistless pattern delineation as well as for in situ processing which may be carried out without opening fabrication apparatus to the outside atmosphere.
Direct patterning of device functional material by ion milling has not proven to be the complete solution since the higher energy required may result in crystallographic damage to remaining material. This complicates or even precludes growth of acceptable overlying material, which sometimes relies upon epitaxy for its intended device function.